We had previously developed techniques for administering test chemicals to earthworms by gavage, and for collecting respiratory carbon dioxide from these animals. More recently we have improved these techniques such that it is possible to accurately determine the dosages actually administered, and to avoid contamination of the trapped carbon dioxide by other (radiolabeled) volatile compounds. We are in the process of developing a method for using fluorinated analogs of radiolabeled test chemicals to elucidate metabolic pathways in which the first step is highly rate-- limiting (so that no intermediates accumulate for identification). The oxidation of o-phthalic acid to CO2 by earthworms appears to be such a pathway. Through the use of 3-, 4- and poly substituted analogues along with radiolabeled, unsubstituted phthalic acid we are able to deduce the sites of oxidative attack on the ring carbon atoms.